User equipment (ue)-triggered handover with early preparation in mobile networks

ABSTRACT

Methods and apparatus are provided for UE-triggered handover and early preparation with coexistence of the network-triggered handover. In one novel aspect, the UE is configured early measurement report configuration, receives an early handover command from the serving base station with a handover candidate cell list, monitors handover triggering conditions for each candidate cell on the handover candidate cell list based on a UE-triggered handover configuration and performs the UE-triggered handover to a candidate cell when the corresponding triggering condition is met for the candidate cell. In one embodiment, the UE receives a network-triggered handover command to a target cell, suspends the UE-triggered handover configuration and performs the network-triggered handover to the target cell. The UE discards the UE-triggered handover configuration upon success of the network-triggered handover and resumes the UE-triggered handover configuration upon failure of the network-triggered handover.

CROSS REFERENCE TO RELATED PATENT APPLICATION(S)

This application claims priority under 35 U.S.C. § 119 from U.S.Provisional Application No. 62/741,663, entitled, “UE-TRIGGERED HANDOVERWITH EARLY PREPARATION IN MOBILE NETWORKS,” filed on Oct. 5, 2018; andU.S. Provisional Application No. 62/754,658, entitled, “Coexistence ofConditional and Normal Handover Mechanisms,” filed on Nov. 2, 2018, thesubject matters of which are incorporated herein by reference.

TECHNICAL FIELD

The disclosed embodiments relate generally to wireless communication,and, more particularly, to UE-triggered handover with early preparation.

BACKGROUND

5G radio access technology will be a key component of the modern accessnetwork. It will address high traffic growth and increasing demand forhigh-bandwidth connectivity. It will also support massive numbers ofconnected devices and meet the real-time, high-reliability communicationneeds of mission-critical applications. New Radio (NR) access technologyis developed to meet a broad range of use cases and requirements for thenext generation. One of the objectives is to support frequency ranges upto 100 GHz. While high frequency system offers larger bandwidth to eachuser equipment (UE) and boosts the throughput, the mobility becomes morechallenging due to higher handover failure rate caused by suddenattenuation of signal strength. In the current NR system, only basicnetwork-controlled handover mechanism is supported. When operating athigh frequency, channel fluctuation is more severe, and a sudden drop ofserving link is observed. In the NR network the links are morevulnerable to blockage by the human body or other objects. Such linkfailures between the serving gNB and the UE prohibits the UE fromreceiving handover commands.

In the NR network-controlled handover, similar to LTE, the target gNBand the timing for handover are determined by the source gNB. The sourcegNB triggers handover by sending HO request to target gNB and afterreceiving an ACK from the target gNB, the source gNB initiates handoverby sending HO command with target cell configuration. The UE sends PRACHto the target cell after RRC reconfiguration is applied with target cellconfiguration. The network-controlled handover allows the network tofully control the UE behavior, but the decisions by source gNB may notbe the best for UE in a fast-changing channel.

Enhancement of the handover procedure in the NR network is required.

SUMMARY

Methods and apparatus are provided for UE-triggered handover and earlypreparation with the coexistence of the network-triggered handover. Inone novel aspect, the UE is configured early measurement reportconfiguration, receives an early handover command from the serving basestation with a handover candidate cell list, monitors handovertriggering conditions for each candidate cell on the handover candidatecell list and performs the UE-triggered handover to a candidate cellwhen the corresponding triggering condition is met for the candidatecell. In one embodiment, the UE receives measurement reportconfiguration from the network, monitoring triggering conditions forcandidate cells on the handover candidate cell list based on aUE-triggered handover configuration. Subsequently, the UE receives anetwork-triggered handover command to a target cell, suspends theUE-triggered handover configuration and performs the network-triggeredhandover to the target cell. In one embodiment, the UE discards theUE-triggered handover configuration upon success of thenetwork-triggered handover. In another embodiment, the UE resumes theUE-triggered handover configuration upon failure of thenetwork-triggered handover. In one embodiment, the early handovercommand includes at least one early handover elements comprising ahandover candidate cell list of handover candidate cells, correspondingtriggering condition for each handover candidate cell on the handovercandidate cell list, and an early handover indication. In anotherembodiment, the UE stores the early handover configuration, suspends theconfiguration when performs the network-triggered handover and performsthe UE-triggered handover if the network-triggered handover fails.

In one embodiment, the UE is configured early measurement reportconfiguration, receives an early handover command from the serving basestation with a handover candidate cell list, monitors handovertriggering conditions for each candidate cell on the handover candidatecell list and performs the UE-triggered handover to a candidate cellwhen the corresponding triggering condition is met for the candidatecell. In one embodiment, the early handover command includes at leastone early handover elements comprising a handover candidate cell list ofhandover candidate cells, corresponding triggering condition for eachhandover candidate cell on the handover candidate cell list, and anearly handover indication. In another embodiment, the handover candidatecell list in the early handover command is based on one or more earlymeasurement reports performed by the UE according to the received earlymeasurement report configuration. In yet another embodiment, the UEsends an early measurement report upon detecting one or more reportingconditions comprising detecting a cell quality of a neighboring cell ishigher than a predefined quality threshold and detecting a cell qualityof a neighboring cell is a predefined offset higher than a cell qualityof the serving cell. When the UE operates in a multi-beam network, andthe cell quality is determined to be over a threshold by meeting atleast one condition comprising a weighted average of qualities of someor all of beams being over the threshold, and a predefined number ofbeams with corresponding measurement being above the threshold. In oneembodiment, the UE performs a random access to the candidate cell andsends a handover complete message to the serving cell upon success ofthe random access. Subsequent to receiving the early handover command, anetwork-triggered handover command to a target cell is received beforethe performing of the UE-triggered handover. The UE performs anetwork-triggered handover to the target cell and performs theUE-triggered handover upon determining a failure of thenetwork-triggered handover. In one embodiment, the UE stores aUE-triggered handover configuration upon receiving the early handovercommand, wherein the UE-triggered handover configuration comprising thehandover candidate cell list of handover candidate cells. In anotherembodiment, the UE-triggered handover configuration further comprises atleast one of corresponding cell identification (ID) of each candidatecell, corresponding C-RNTI, and corresponding security algorithmidentifier of each candidate cell. In another embodiment, upon receivinga network-triggered handover command towards a target cell, the UEsuspends the UE-triggered handover configuration. In yet anotherembodiment, the UE discards the UE-triggered handover configuration upona successful completion of a network-triggered handover to the targetcell, and wherein the UE reassumes the suspended UE-triggered handoverconfiguration upon a failure of a network-triggered handover to thetarget cell.

In another novel aspect, the source base station preparing an earlyhandover command for a UE, wherein the early handover command comprisesat least one early handover elements comprising a handover candidatecell list of handover candidate cells, corresponding triggeringcondition for each handover candidate cell on the handover candidatecell list, and an early handover indication. The source base stationsends the early handover command to the UE and prepares one or morehandover candidate cells for a UE-triggered handover based on one ormore triggering events. In one embodiment, the source base stationperforms sequence number (SN) STATUS TRANSFER to the handover candidatecells. The source base station sends a CONDITIONAL HANDOVER REQUEST toeach of the candidate cells and receives corresponding CONDITIONALHANDOVER RESPONSE.

Other embodiments and advantages are described in the detaileddescription below. This summary does not purport to define theinvention. The invention is defined by the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, where like numerals indicate like components,illustrate embodiments of the invention.

FIG. 1 illustrates an exemplary wireless network 100 with early handoverin accordance with embodiments of the current invention.

FIG. 2 illustrates exemplary diagrams for a moving UE changing cells ina multi-beam frequency system and performing a normal handover vs. aUE-triggered handover in accordance with embodiments of the currentinvention.

FIG. 3 illustrates an exemplary diagram for an early measurement reportin accordance with embodiments of the current invention.

FIG. 4 illustrates a details diagram of mobility-related events atdifferent time instants for the UE-triggered handover in accordance withembodiments of the current invention.

FIG. 5 illustrates an exemplary diagram for a UE-triggered handover withearly preparation in accordance with embodiments of the currentinvention.

FIG. 6 illustrates an exemplary flow diagram for an early handovercommand with a success UE-triggered handover procedure in accordancewith embodiments of the current invention.

FIG. 7 illustrates an exemplary flow diagram for a UE performing legacyhandover after early handover preparation in accordance with embodimentsof the current invention.

FIG. 8 illustrates an exemplary flow diagram of a UE fall back toperform the UE-triggered handover after the failure of thenetwork-triggered handover in accordance with embodiments of the currentinvention.

FIG. 9A illustrates an exemplary diagram for a UE configured with boththe UE-triggered handover and the network-triggered handover andsucceeds in the network-triggered handover in accordance withembodiments of the current invention.

FIG. 9B illustrates an exemplary diagram for a UE configured with boththe UE-triggered handover and the network-triggered handover andsucceeds in the UE-triggered handover in accordance with embodiments ofthe current invention.

FIG. 10 illustrates an exemplary flow diagram for a network-triggeredhandover to a target cell being early prepared by an early handovercommand in accordance with embodiments of the current invention.

FIG. 11 illustrates an exemplary flow diagram for a network-triggeredhandover to a target cell not prepared in accordance with embodiments ofthe current invention.

FIG. 12 illustrates an exemplary diagram for the UE to perform the earlyhandover preparation and UE-triggered handover in accordance withembodiments of the current invention.

FIG. 13 illustrates an exemplary diagram for the source base station toperform the early handover preparation and UE-triggered handover inaccordance with embodiments of the current invention.

DETAILED DESCRIPTION OF PREFERRED IMPLEMENTATIONS

Reference will now be made in detail to some embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings.

FIG. 1 illustrates an exemplary wireless network 100 with early handoverin accordance with embodiments of the current invention. Wirelesscommunication system 100 includes one or more wireless networks each ofthe wireless communication network has fixed base infrastructure units,such as receiving wireless communications devices 102 103, and 104,forming wireless networks distributed over a geographical region. Thebase unit may also be referred to as an access point, an accessterminal, a base station, a Node-B, an eNode-B, or by other terminologyused in the art. Each of the receiving wireless communications devices102, 103, and 104 serves a geographic area. Backhaul connections 113,114 and 115 connect the non-co-located receiving wireless communicationsdevices, such as 102, 103, and 104. These backhaul connections can beeither ideal or non-ideal.

A wireless communications device 101 in wireless network 100 is servedby base station 102 via uplink 111 and downlink 112. Other wirelesscommunications devices 105, 106, 107, and 108 are served by differentbase stations. Wireless communications devices 105 and 106 are served bybase station 102. Wireless communications device 107 is served by basestation 104. Wireless communications device 108 is served by basestation 103.

In one embodiment, wireless communication network 100 is an OFDMA/NRsystem comprising base stations 102, 103 and 104, and mobile stations,such as wireless station/UE 101. The base stations can be eNBs, gNBs,access points (Aps) or other wireless stations. In other embodiments,the base station can be multi-beam stations. Base station 102 has multibeams such as 102A and 102B. Similarly, base station 103 has multi beamssuch as 103A and 103B, and base station 104 has multi beams such as 104Aand 104B. In the applications, each base station serves multiplewireless communications devices that periodically transmit packets. Anetwork entity, such as a wireless controller 109 is connected to basestations such as base station 102, 103, and 104, via links of 116, 117,and 118.

In one novel aspect, early handover/conditional handover and earlyhandover preparation are performed by the network. Conventionally, in NRnetwork-controlled handover, similar to LTE, the target gNB and thetiming for handover are determined by the source gNB. The source gNBtriggers handover by sending HO request to a target gNB and afterreceiving an ACK from the target gNB, the source gNB initiates handoverby sending HO command with target cell configuration. The UE sends PRACHto the target cell after RRC reconfiguration is applied with target cellconfiguration. The network-controlled handover allows the network tofully control the UE behavior, but the decisions by source gNB may notbe the best for UE in a fast-changing channel. To improve mobilityrobustness, the early handover/conditional handover mechanism isprovided. In early handover, the source gNB also sends handover commandto UE, but the command is associated with some conditions. The UEperforms handover to the indicated target gNB not right upon receivingthe handover command but when the conditions are met. If both normal andconditional handover mechanisms are to be configured, additionalprocedures need to be introduced to ensure proper coexistence of twohandover procedures run on a single UE.

In one novel aspect, early handover and early preparation are performedby the UE to improve the handover procedure for network 100. Initially,UE 101 is connected to source gNB 102 via uplink 111 and downlink 112.In one embodiment, UE 101 receives measurement report configuration fromthe network. The measurement report includes early measurementconfigurations.

UE performs measurements. As UE 101 moves, at 101-B, UE 101 may send ameasurement report to the source gNB 102. Source gNB 102 makes earlyhandover decisions and send early handover command to UE 101. In oneembodiment, source gNB 102 prepares candidate cells, such as gNB 103 andgNB 104 for handover. The early handover preparation for gNB 103 and gNB104 includes performs admission control for the potential handover. Theearly handover command to UE 101 may include a list of candidatehandover cells, such as gNB 103 and gNB 104. In another embodiment, theearly handover command may also include corresponding handoverconditions for each candidate cells. UE 101 monitors these handoverconditions. As UE 101 continues moving, at 101-C, UE 101 detects ahandover condition for the early handover. UE 101 triggers aUE-triggered handover procedure to gNB 104. At 101-D, UE 101successfully connected to the gNB 104.

FIG. 1 further shows simplified block diagrams of wireless stations 101and base station 102 in accordance with the current invention.

Base station 102 has an antenna 126, which transmits and receives radiosignals. A RF transceiver module 123, coupled with the antenna, receivesRF signals from antenna 126, converts them to baseband signals and sendsthem to processor 122. RF transceiver 123 also converts receivedbaseband signals from processor 122, converts them to RF signals, andsends out to antenna 126. Processor 122 processes the received basebandsignals and invokes different functional modules to perform features inbase station 102. Memory 121 stores program instructions and data 124 tocontrol the operations of base station 102. Base station 102 alsoincludes a set of control modules, such as handover circuit 125 thatcarry out functional tasks to communicate with wireless communicationsdevices.

Wireless communications device 101 has an antenna 135, which transmitsand receives radio signals. A RF transceiver module 134, coupled withthe antenna, receives RF signals from antenna 135, converts them tobaseband signals and sends them to processor 132. RF transceiver 134also converts received baseband signals from processor 132, convertsthem to RF signals, and sends out to antenna 135. Processor 132processes the received baseband signals and invokes different functionalmodules to perform features in mobile station 101. Memory 131 storesprogram instructions and data 136 to control the operations of mobilestation 101.

Wireless communications device 101 also includes a set of controlmodules that carry out functional tasks. a measurement report circuit191 receives an early measurement report configuration that configuresUE early measurement reports from a serving cell. A handover commandreceiver 192 receives an early handover command from the serving basestation, wherein the early handover command comprising a handovercandidate cell list of handover candidate cells, correspondingtriggering condition for each handover candidate cell on the handovercandidate cell list, and an early handover indication. A conditionmonitor 193 monitors triggering conditions for candidate cells on thehandover candidate cell list based on a UE-triggered handoverconfiguration. A handover circuit 194 upon receiving a network-triggeredhandover command to a target cell suspends the UE-triggered handoverconfiguration.

FIG. 2 illustrates exemplary diagrams for a moving UE changing cells ina multi-beam frequency system and performing a normal handover vs. aUE-triggered handover in accordance with embodiments of the currentinvention. UE 201 is connected to a source gNB 202 in a multi-beamfrequency system. gNB 202 has multi beams such as 202A and 202B.Neighboring cell gNB 203 has multi beams such as 203A and 203B.Neighboring cell gNB 204 has multi beams such as 204A and 204B. In anormal situation, as UE 201 moves from 201-A to 201-B to 201-C and to201-D, as in diagram 210, UE 201 detects signals for beam 202A, 202B,204A and 204B as curves 211, 212, 213, and 214, respectively. In aconventional way, UE 201 handover to gNB 204 at 201-C where 204A beamquality of 213 of gNB 204 is offset better than 201B beam quality of212. UE 201 at 201C handover to gNB 204 and connected beam 204A. As UEcontinues moving, at 201-D, UE 201 may handover to beam 204B of gNB 204.

However, as in diagram 230, as UE 201 moves from 201-A to 201-B to 201-Cand to 201-D, UE 201 detects signals for beam 202A, 202B, 204A and 204Bas curves 231, 232, 233, and 234, respectively. Beam 202B, as shown incurve 232, has a sudden drop of quality at 251. As result of this, theconventional network-triggered handover may fail because the UE may notbe able to receive the handover command as the signal quality of 202Bdrops suddenly. In one novel aspect, early handover and early handoverpreparation may be performed. As UE 201 moves 201-B, the UE may triggerearly measurement report based on an early measurement reportconfiguration. The early measurement report may include measurements forgNB 203. Subsequently, a conditional handover command/early handovercommand is received from the network, with a candidate list includes gNB203 and gNB 204. In one embodiment, when the network-triggered handoverto gNB 204 failed, UE 201 may perform a UE-triggered handover based onthe early handover command received. UE 201 may perform the UE-triggeredhandover to gNB 204 based on the triggering condition being met eventhough the network-triggered handover command is lost or failed.

FIG. 3 illustrates an exemplary diagram for an early measurement reportin accordance with embodiments of the current invention. In oneembodiment, early measurement report configuration is received by theUE. The early measurement configuration may configure a periodicmeasurement report for the UE and/or an event triggered measurementreport. Measurement report trigger 301 may include one or moreconditions such as 311 and 312. At condition 311, the UE detects aneighboring cell that is above a threshold. At condition 312, the UEdetects a neighboring cell that is offset better than the serving cell.The offset used to trigger earlier measurement report can be lower thanthat for a normal measurement, such as an A3 event. Moreover, whencomparing a multi-beam cell with another one or against a threshold, acell is represented by a single cell quality value derived from themeasurements from multiple beams. The condition for the cell quality ina multi-beam cell can be determined in one or more predefined mannerssuch as 321 and 322. Measurement 321 measures a cell quality by aweighted average of qualities of some or all of beams. Measurement 322measures a cell quality by counting the number of beams with qualityhigher than a threshold. The measurement report may contain neighboringcell IDs, cell quality, beam-level quality, or other measurementsconfigured by the network.

FIG. 4 illustrates a details diagram of mobility-related events atdifferent time instants for the UE-triggered handover in accordance withembodiments of the current invention. UE 400 is connected to source gNB401 of cell-1 at time 400-T1. The network indicates gNB 402 of cell-2and gNB 403 of cell-3 are potential target/candidate cells. In oneembodiment, UE 400 receives an early handover command from the networkwith a candidate cell list of cell-2 and cell-3. In another embodiment,the early handover command also includes handover conditions for cell-2and cell-3. At time 400-T1, UE 400 detects handover condition for cell-2is met. UE 400 performs handover to gNB 402 of cell-2. At time 400-T3,the network indicates gNB 404 of cell-4 as potential target cell orcandidate cell. In one embodiment, UE 400 receives an early handovercommand from the network with a candidate cell list of cell-4 andoptionally includes the handover triggering condition for cell-4. At400-T4, UE 400 detects handover condition for cell-4 is met. UE 400performs handover to gNB 404 of cell-4. At time 400-T5, the networkindicates gNB 405 of cell-5 and gNB 406 of cell-6 are potentialtarget/candidate cells. In one embodiment, UE 400 receives an earlyhandover command from the network with a candidate cell list of cell-5and cell-6 and optionally includes handover conditions for cell-5 andcell-6. At time 400-T6, UE 400 detects handover condition for cell-5 ismet. UE 400 performs handover to gNB 405 of cell-5.

FIG. 5 illustrates an exemplary diagram for a UE-triggered handover withearly preparation in accordance with embodiments of the currentinvention. At step 501, the UE receives measurement report configurationfrom the network. In one embodiment, the measurement reportconfiguration includes early measurement report configuration for theUE-triggered handover. At step 502, the UE performs measurement ofneighboring cell according to the measurement report configuration. Inone embodiment, the measurement report is periodic 551. In anotherembodiment, the measurement report is event triggered 552. At step 511,the UE sends the measurement. In some embodiments, the sending of themeasurement report is triggered by one or more triggering conditionssuch as 561 and 562. In 561, the measurement report is sent upondetecting the neighboring cell quality is higher than a predefinedthreshold. In one embodiment, the predefined threshold is lower than forthe conventional measurement report. In 562, the measurement report issent upon detecting the neighboring cell quality is an offset betterthan the serving cell quality. The above conditions are similar to thetriggering conditions of measurement events, but the parameters can bedifferent. For example, the offset used to trigger earlier measurementreport can be lower than that for a normal A3 event. Moreover, whencomparing a multi-beam cell with another one or against a threshold, acell is represented by a single cell quality value derived from themeasurements from multiple beams. The measurement report may containneighboring cell IDs, cell quality, beam-level quality, othermeasurement configured by the network. If step 511 determines yes, theUE sends the measurement report at step 512.

Upon receiving the measurement report, the network makes early handoverdecisions based on the measurement report. Upon receiving earlymeasurement report, the serving gNB evaluates whether the reportedneighbor cells can be good candidates. If a cell is considered aspotential target for handover in the future, the serving gNB preparesthe target by sending early handover request carrying UE context. Ifpositive response is received, serving gNB sends an early handovercommand to UE, indicating potential target cells that have beenprepared. At step 521, the UE receives early handover command from thenetwork. The handover command includes a handover configuration. Thenetwork upon sending the early handover command, prepares one or morecandidate cells included early handover command.

In one embodiment, the UE upon receiving the early handover command,stores the handover configuration included in the early handovercommand. The early handover command includes a handover cell list of oneor more candidate cells. In one embodiment, the early handover commandalso includes corresponding handover conditions for each candidate cell.In another embodiment, the handover condition may be a general conditionapplying to all or multiple cells.

The proposed mechanism allows UE to determine the exact triggering pointof handover. The UE may evaluate and chooses a target cell from thecandidates indicated by the earlier handover command. The serving cellfalls below a threshold for a given duration. The conditions include (1)UE experience RLF (i.e. serving cell falls below out-of-sync thresholdand RLF timer expires), (2) UE cannot decode control channel for a givenduration that is shorter than RLF timer, or (3) serving cell falls belowa threshold higher than the out-of-sync threshold for a given duration.The UE detects a neighboring cell satisfying the conditions for A3-likeevent, such as the neighboring cell is offset better than the servingcell within a given duration. In one embodiment, the triggeringcondition of UE-triggered handover is up to UE implementation. Inanother embodiment, the network provides some suggestions in the earlyhandover command. If UE possesses a list of multiple potential targetcells, it is up to UE implementation which cell to be chosen as thetarget cell. For example, the UE may choose the target cell with highestRSROP or RSRQ or the first candidate target cell that is still valid.

The network may also indicate whether UE should continue to operatelegacy handover after receiving early handover command, and the UE mayadjust its handover triggering mechanism accordingly. For example, if UEdoes not operate legacy handover, the (UE-triggered) handover triggeringcondition should be based on comparing serving and neighboring cellquality (e.g., A3-like), or based on serving cell quality but thethreshold should not be too low. In contrast, if legacy handover isstill operated, UE may adopt either triggering mechanism

In another embodiment, the early handover command further includes othercell information 571, such as the cell IE, the C-RNTI, and the candidategNB security algorithm identifiers for the selected security algorithms.At step 531, the UE monitors the early handover conditions anddetermines if one or more conditions are met. In one embodiment, theconditions are evaluated based on one or more criteria, such as 581 and582. In 581, the condition is met upon detecting the neighboring cellquality is higher than a predefined threshold. In one embodiment, thepredefined threshold is lower than for the conventional measurementreport. In 582, the condition is met upon detecting the neighboring cellquality is an offset better than the serving cell quality. If step 531determines yes, the UE, at step 532, starts a random access to thecorresponding candidate cell.

In one novel aspect, UE-triggered handover is performed to improve thehandover procedure. The UE performs periodic measurements on serving andneighboring cells and delivers measurement report. Based on themeasurement report, the serving gNB sends earlier handover command toUE, indicating potential target cells for a UE. Serving gNB alsoprepares the potential target cells, i.e., delivers UE context to thesecells. The UE performs handover evaluation on the target cells. When thehandover criterion is fulfilled for a neighbor cell, the UE performsrandom access toward the selected target cell. After the RACH completes,the UE sends handover complete message to the new serving gNB.

FIG. 6 illustrates an exemplary flow diagram for an early handovercommand with a success UE-triggered handover procedure in accordancewith embodiments of the current invention. UE 601 is connected to sourcegNB 602 with packet data 611. At step 612, the UE performs RRMmeasurement. In one embodiment, the measurement is performed based onmeasurement report configuration received from the network. At step 621,the UE detects an early measurement trigger event. At step 622, the UEsends an early measurement report. At step 623, source gNB 602 madeearly handover decision based on the received measurement report. SourcegNB 602 prepares candidate cells for handover. At step 624, source gNB602 sends an early handover request to candidate-1 gNB 603. At step 626,gNB 603 upon receiving the early handover request, prepares for thehandover, such as admission control. At step 626, gNB 603 sends an earlyhandover response to source gNB 602. At step 627, gNB 602 sends an earlyhandover command to UE 601. At step 631, the UE detects an earlymeasurement trigger event. At step 632, the UE sends an earlymeasurement report. At step 633, source gNB 602 made early handoverdecision based on the received measurement report. Source gNB 602prepares candidate cells for handover. At step 624, source gNB 602 sendsan early handover request to candidate-1 gNB 604. At step 636, gNB 604upon receiving the early handover request, prepares for the handover,such as admission control. At step 636, gNB 604 sends an early handoverresponse to source gNB 602. At step 637, gNB 602 sends an early handovercommand to UE 601. Optionally, at step 641, gNB 602 sends an earlyhandover cancellation to gNB 603. At step 642, gNB 603 discards thehandover resources for UE 601.

The UE upon receiving one or more early handover commands, monitorshandover triggering conditions. At step 651, UE 601 makes early handoverdecisions. Upon determining one or more triggering conditions are met,the UE performs the UE-triggered handover. At step 651, UE 601 performssynchronization and RA with gNB 604. At step 652, UE 601 sends an earlyhandover complete message to gNB 604. In one embodiment the earlyhandover complete message is an RRC reconfiguration complete message. Atstep 653, gNB 604 sends sequence number (SN) status transfer requestmessage to source gNB 602. At step 654, gNB 602 sends SN status transferresponse message. At step 655, gNB 602 performs data forwards to gNB604. At step 661, UE 601 transmits and receives packet data with gNB604.

A UE supporting UE-triggered handover may operates legacy handoverprocedure at the same time, that is, it still sends legacy measurementreports and receives legacy handover command. In this case, UE mayswitch from one kind of handover procedure to another. In most cases,early measurement report is sent earlier than legacy measurement report.If UE-triggered handover is set to be triggered based on serving cellquality, after receiving conditional handover command, UE may stillevaluate neighboring cells based on legacy measurement eventconfigurations. Moreover, if the target cell chosen later is prepareddue to early handover command, source gNB needs not to send handoverrequest to target gNB.

FIG. 7 illustrates an exemplary flow diagram for a UE performing legacyhandover after early handover preparation in accordance with embodimentsof the current invention. UE 701 is connected to source gNB 702 withpacket data 711. The network also has gNB 703 and 704. At step 712, theUE performs RRM measurement. In one embodiment, the measurement isperformed based on measurement report configuration received from thenetwork. At step 721, the UE detects an early measurement trigger event.At step 722, the UE sends an early measurement report. At step 723,source gNB 702 made early handover decision based on the receivedmeasurement report. Source gNB 702 prepares candidate cells forhandover. At step 724, source gNB 702 sends an early handover request tocandidate-1 gNB 703. gNB 703 upon receiving the early handover request,prepares for the handover, such as admission control. At step 726, gNB703 sends an early handover response to source gNB 702. At step 727, gNB702 sends an early handover command to UE 701.

At step 731, UE 701 detects legacy measurement trigger event. At step732, the UE 701 sends legacy measurement report. Source gNB 702determines a network-triggered handover. At step 733, source gNB 702sends an RRC reconfiguration to UE 701. In one embodiment, source gNB702 chooses the target gNB as gNB 702. Since gNB 702 is a candidate cellfor the early handover, there is no need for preparation for the legacyhandover. At step 741, source gNB 702 sends SN status transfer to gNB703.

Upon receiving the RRC reconfiguration message, at step 751, UE 601performs synchronization and RA with gNB 703. At step 752, UE 701 sendsan early handover complete message to gNB 703. At step 755, gNB 702performs data forwards to gNB 703. At step 756, gNB 703 sends UE contentrelease to source gNB 702. At step 761, UE 701 transmits and receivespacket data with gNB 703.

In one embodiment, the UE-triggered handover may also serve as a failurerecovery mechanism for the network-triggered handover. Assuming theUE-triggered handover being set to be triggered based on serving cellquality and UE still evaluates legacy measurement events, the UEreceives conditional handover conditional command for a candidate gNB#2. Subsequently, the UE sends legacy measurement report for a targetgNB #1 when it moves toward cell edge. If the UE experiences suddenchannel degradation, it fails to receive legacy handover command fortarget gNB #1. Instead of triggering RRC reestablishment due to handoverfailure, the UE with conditional handover target gNB #2 can perform RACHand send UE-triggered handover indication to target gNB #2. The methodleads to faster recovery from handover failure. In another scenario, theearly handover command may include both gNB #1 and gNB #2. When thenetwork-triggered handover to gNB #1 failed due to the link failure inthe source gNB and missing the handover command, the UE may stillperform the UE-triggered handover to gNB #1 as well.

FIG. 8 illustrates an exemplary flow diagram of a UE fall back toperform the UE-triggered handover after the failure of thenetwork-triggered handover in accordance with embodiments of the currentinvention. UE 801 is connected to source gNB 802 with packet data 811.The network also has gNB 803 and 804. At step 821, the UE detects anearly measurement trigger event. At step 822, the UE sends an earlymeasurement report. At step 823, source gNB 802 made early handoverdecision based on the received measurement report. Source gNB 802prepares candidate cells for handover. At step 824, source gNB 802 sendsan early handover request to candidate-1 gNB 804. gNB 804 upon receivingthe early handover request, prepares for the handover, such as admissioncontrol. At step 826, gNB 804 sends an early handover response to sourcegNB 802. At step 827, gNB 802 sends an early handover command to UE 801.At step 831, UE 801 detects legacy measurement trigger event. At step832, the UE 801 sends legacy measurement report. Source gNB 802determines a network-triggered handover. At step 833, source gNB 802sends an RRC reconfiguration to UE 801. However, the RRC reconfigurationat step 833 is failed.

In one embodiment, upon the failure of the network-triggered handover,the UE performs the UE-triggered handover at step 841. At step 851, UE801 performs synchronization and RA with gNB 804. At step 852, UE 801sends a UE-triggered handover indication to gNB 804. At step 853, gNB804 sends sequence number (SN) status transfer request message to sourcegNB 802. At step 854, gNB 802 sends SN status transfer response message.At step 855, gNB 802 performs data forwards to gNB 804. At step 856, gNB804 sends UE content release to source gNB 802. At step 861, UE 801transmits and receives packet data with gNB 803.

In one novel aspect, the UE-triggered handover, or conditional handover(CHO) coexists with the network-triggered handover, or the legacyhandover. The UE-triggered/CHO is a mobility procedure for UEs inconnected mode, where a handover command sent by source gNB isassociated with potential target gNB(s) and certain conditions, and theUE performs handover to a target gNB once the conditions are met. Thehandover command is used for both the UE-triggered handover and thenetwork-triggered handover. The handover command sent by source gNB toUE, indicating the target or candidate cell. The command can be carriedby RRC messages, such as the RRC connection reconfiguration. The commandcan indicate a normal handover, which is to be performed immediately, ora conditional handover, which is performed when the condition is met.

FIG. 9A illustrates an exemplary diagram for a UE configured with boththe UE-triggered handover and the network-triggered handover andsucceeds in the network-triggered handover in accordance withembodiments of the current invention. At time 900-A, UE 900 is connectedto source gNB 901 at step 931 in a wireless network, which also includesneighboring gNB 902 and gNB 903. At time 900-B, UE 900 moves and anearly handover command is received from source gNB 901 at step 932. Theearly handover command includes candidate cells served by gNB 902 andgNB 903. At time 900-C, UE 900 receives network-triggered handovercommand to gNB 903, at step 933, and performs the handover to gNB 903.At time 900-D, UE 900 successfully connected to gNB 903 at step 934. Inone embodiment, the UE performs the network-triggered handover to gNB903, which is also on the UE-triggered candidate cell list. Uponsucceeds in the network-triggered handover, UE 900 discards the earlyhandover configuration.

FIG. 9B illustrates an exemplary diagram for a UE configured with boththe UE-triggered handover and the network-triggered handover andsucceeds in the UE-triggered handover in accordance with embodiments ofthe current invention. At time 900-A, UE 900 is connected to source gNB901 at step 931 in a wireless network, which also includes neighboringgNB 902 and gNB 903. At time 900-B, UE 900 moves and an early handovercommand is received from source gNB 901 at step 932. The early handovercommand includes candidate cells served by gNB 902 and gNB 903. At time900-C, UE 900 receives network-triggered handover command to gNB 903, atstep 933. However, before UE 900 completes random access to gNB 903, thepath to source gNB 902 experiences blockage by a building. In normalhandover, this results in radio link failure (RLF).

To prevent UE from triggering RLF in such cases, the UE may suspend theUE-triggered handover configurations instead of discarding them. In oneembodiment, the network-triggered handover is the major mobilitymechanism, and the UE-triggered handover is a failure recoverymechanism. For example, the early handover condition may be set as beingthe source gNB falls below a threshold and the candidate gNB is aboveanother threshold. In the current example, it is satisfied for gNB 902when UE fails to connect to gNB 903 but is far away from source gNB 901.However, if the UE-triggered configuration for gNB 902 is stillavailable, the UE realizes that pre-configured conditions forUE-triggered handover to gNB 902 are indeed satisfied, and gNB 902becomes the target for the UE-triggered handover. At step 944, UE 900succeeds in UE-triggered handover to gNB 902.

When a candidate gNB is included in a CHO command, it means thecandidate gNB has been prepared by the source gNB for the UE. Thecandidate gNB possesses the UE context and has reserved RACH resourcesfor the UE. If the candidate gNB is then selected by source gNB as thetarget gNB for normal handover, the source gNB needs not to send anotherHANDOVER REQUEST message to the target gNB. Otherwise, if the selectedtarget gNB is never prepared, the source gNB needs to send a HANDOVERREQUEST message to the target gNB and wait for ACK before sendinghandover command to UE. When a target gNB is selected, either by sourcegNB for the network-triggered handover or by UE for conditionalhandover, there may be other candidate gNBs still holding UE context andresources for CHO. These candidate gNBs can release the resourcesautonomously. Alternatively, if the source gNB initiates a normalhandover, the source gNB may send notification to these candidate gNBswhen the handover command is HARQ acknowledged.

FIG. 10 illustrates an exemplary flow diagram for a network-triggeredhandover to a target cell being early prepared by an early handovercommand in accordance with embodiments of the current invention. UE 1001is connected to source gNB 1002 with packet data 1011. The network alsohas gNB 1003 and 1004. Data path 1012 is established between source gNB1002 and Core Access and Mobility Management Function (AMF) 1005 anduser plan function (UPF) 1006. At step 1021, UE 1001 receivesmeasurement report configuration from the network. The measurementreport configuration includes early measurement report configuration andmeasurement events, such as the A3-like event with lower threshold. Thenetwork performs early handover preparation to candidate cells 1020.Procedure 1020 includes steps. UE 1001 sends early/conditionalmeasurement report when corresponding configured event is triggered.Source gNB 1002 decides that a conditional handover is needed. SourcegNB 1002 sends CONDITIONAL HANDOVER REQUEST to one or more candidategNBs, such as gNB 1003 and/or gNB 1004. The selected one or morecandidate gNBs performs admission control. The one or more candidategNBs sends CONDITIONAL HANDOVER RESPONSE to serving gNB 1002 indicatingthe requested is being accepted or rejected.

At step 1028, source gNB 1002 sends an early handover command to UE1001. In one embodiment, the one or more candidate gNBs prepare handoverand the response message includes a transparent container to be sent tothe UE carrying necessary information, such as the dedicated RACHpreamble. Source gNB sends the early handover/CHO command to UE 1001. Inone embodiment, the early handover command includes information obtainedfrom the candidate cells, conditions for triggering the UE-triggeredhandover, such as threshold and corresponding time-to-trigger timer, andconditions for UE to exit the triggering condition, such as predefinedoffsets. In one embodiment, the early handover command may also includean early handover indication, which indicates the message is an earlyhandover command. At step 1031, UE 1001 stores the received handoverconfiguration in the early handover command.

Subsequently, UE 1001 detects legacy measurement trigger event. At step1032, UE 1001 sends measurement report. Source gNB decides that ahandover is needed and discovers that the target gNB 1003 is alreadyprepared for conditional handover. Therefore, gNB 1002 sends SN statustransfer to gNB 1003 at step 1041 and optionally at step 1042 forwardeddata to gNB 1003. At step 1033, source gNB 1002 sends a handover commandto UE 1001. At step 1034, UE 1001 suspends the CHO configuration.

At step 1051, UE 1001 performs synchronization and RA with gNB 1003. Atstep 1052, UE 1001 sends an RRC reconfiguration complete to gNB 1003. Atstep 1035, UE 1001 discards the CHO configuration. At step 853, gNB 804sends sequence number (SN) status transfer request message to source gNB802. At step 1061, gNB 1003 sends a PATH SWITCH REQUEST message to AMF1005 to inform that the UE has changed cell. At step 1062, AMF 1005sends a MODIFY BEARER REQUEST message to UPF 1006. At step 1063, UPF1006 switches the downlink data path to the target side via 1066. UPF1006 sends one or more END MAKER packets 1065 on the old path to sourcegNB 1002. At step 1063, UPF 1006 sends a MODIFY BEARER RESPONSE to AMF1005. AMF 1005, at step 1064, confirms the PATH SWITCH REQUEST with thePATH SWITCH REQUEST ACK message. At step 1081, gNB 1003 sends UE CONTEXTRELEASE and informs the success of handover to gNB 1002. gNB 1002, atstep 1082 releases resources including the radio and C-plane relatedresources associated with the UE context. At step 1083, gNB 1003discards the handover configuration.

FIG. 11 illustrates an exemplary flow diagram for a network-triggeredhandover to a target cell not prepared in accordance with embodiments ofthe current invention. UE 1101 is connected to source gNB 1102 withpacket data 1111. The network also has gNB 1103 and 1104. Data path 1112is established between source gNB 1102 and AMF 1105 and UPF 1106. Atstep 1121, UE 1101 receives measurement report configuration, similar tothat as in 1021, from the network. At step 1122, UE 1101 sends an earlymeasurement report to gNB 1102. At step 1123, gNB 1102 sends early HOrequest to gNB 1103. At step 1125, gNB 1103 prepares handover andperforms admission control. At step 1126, gNB 1103 sends HO response togNB 1102.

At step 1128, source gNB 1102 sends an early handover command to UE1101. At step 1131, UE 1101 stores the received handover configurationin the early handover command.

Subsequently, UE 1101 detects legacy measurement trigger event. At step1132, UE 1101 sends measurement report. Source gNB decides that ahandover is needed and discovers that the target gNB 1104 is notprepared for the handover. At step 1133, gNB 1102 sends HO request togNB 1104. At step 1135, gNB 1104 performs admission control. At step1136, gNB 1104 sends HO response to gNB 1102. At step 1133, source gNB1102 sends a handover command to UE 1101. At step 1134, UE 1101 suspendsthe CHO configuration.

At step 1151, UE 1101 performs synchronization and RA with gNB 1104. Atstep 1152, UE 1101 sends an RRC reconfiguration complete to gNB 1104. Atstep 1135, UE 1101 discards the CHO configuration. At step 1141, gNB1102 sends sequence number (SN) status transfer to gNB 1104. At step1142, gNB 1102 performs data forwarding to gNB 1104. Path modification1170 are performed similar to that in steps 1061 to 1065. At step 1181,gNB 1104 sends UE CONTEXT RELEASE and informs the success of handover togNB 1102. gNB 1102, at step 1182 releases resources including the radioand C-plane related resources associated with the UE context. At step1183, gNB 1103 discards the handover configuration.

FIG. 12 illustrates an exemplary diagram for the UE to perform the earlyhandover preparation and UE-triggered handover in accordance withembodiments of the current invention. At step 1201, the UE receives anearly measurement report configuration that configures UE earlymeasurement reports from a serving cell in a wireless network. At step1202, the UE receives an early handover command from the serving basestation, wherein the early handover command comprising at least oneearly handover elements comprising a handover candidate cell list ofhandover candidate cells, corresponding triggering condition for eachhandover candidate cell on the handover candidate cell list, and anearly handover indication. At step 1203, the UE monitors triggeringconditions for candidate cells on the handover candidate cell list basedon a UE-triggered handover configuration. At step 1204, subsequently,the UE receives a network-triggered handover command to a target cell.At step 1205, the UE suspends the UE-triggered handover configuration.At step 1206, the UE performs a network-triggered handover to the targetcell.

FIG. 13 illustrates an exemplary diagram for the source base station toperform the early handover preparation and UE-triggered handover inaccordance with embodiments of the current invention. At step 1301, thesource base station prepares an early handover command for a UE, whereinthe early handover command comprising at least one early handoverelements comprising a handover candidate cell list of handover candidatecells, corresponding triggering condition for each handover candidatecell on the handover candidate cell list, and an early handoverindication. At step 1302, the source base station sends the earlyhandover command to the UE. At step 1303, the source base stationprepares one or more handover candidate cells for a UE-triggeredhandover based on one or more triggering events.

Although the present invention has been described in connection withcertain specific embodiments for instructional purposes, the presentinvention is not limited thereto. Accordingly, various modifications,adaptations, and combinations of various features of the describedembodiments can be practiced without departing from the scope of theinvention as set forth in the claims.

1. A method comprising: receiving an early measurement reportconfiguration that configures UE early measurement reports from aserving cell by a user equipment (UE) in a wireless network; receivingan early handover command by the UE from the serving base station,wherein the early handover command includes at least one early handoverelements comprising a handover candidate cell list of handover candidatecells, corresponding triggering condition for each handover candidatecell on the handover candidate cell list, and an early handoverindication; monitoring triggering conditions for candidate cells on thehandover candidate cell list based on a UE-triggered handoverconfiguration; subsequently, receiving a network-triggered handovercommand to a target cell; suspending the UE-triggered handoverconfiguration; and performing a network-triggered handover to the targetcell.
 2. The method of claim 1, wherein the handover candidate cell listin the early handover command is based on one or more early measurementreports performed by the UE according to the received early measurementreport configuration.
 3. The method of claim 1, wherein the UE sends anearly measurement report upon detecting one or more reporting conditionscomprising detecting a cell quality of a neighboring cell being higherthan a predefined quality threshold and detecting a cell quality of aneighboring cell being an predefined offset higher than a cell qualityof the serving cell.
 4. The method of claim 3, wherein the UE operatesin a multi-beam network, and the cell quality is determined be over athreshold by meeting at least one condition comprising a weightedaverage of qualities of some or all of beams being over the threshold,and a predefined number of beams with corresponding measurement beingabove the threshold.
 5. The method of claim 1, further comprising:storing the UE-triggered handover configuration upon receiving the earlyhandover command, wherein the UE-triggered handover configurationcomprises the handover candidate cell list of handover candidate cells.6. The method of claim 1, wherein the UE-triggered handoverconfiguration further comprising at least one of corresponding cellidentification (ID) of each candidate cell, corresponding C-RNTI, andcorresponding security algorithm identifier of each candidate cell. 7.The method of claim 1, wherein the UE discards the UE-triggered handoverconfiguration upon a successful completion of a network-triggeredhandover to the target cell.
 8. The method of claim 1, wherein the UEresumes the suspended UE-triggered handover configuration upon a failureof a network-triggered handover to the target cell.
 9. The method ofclaim 8, further comprising performing a UE-triggered handover to acandidate cell on the handover candidate cell list when correspondingone or more triggering conditions for the candidate cell are satisfied.10. The method of claim 9, wherein the performing of UE-triggeringhandover to the candidate cell involves: performing a random access tothe candidate cell; and sending a handover complete message to theserving cell upon success of the random access.
 11. A method comprising:preparing an early handover command for a user equipment (UE) by aserving base station, wherein the early handover command comprising atleast one early handover elements comprising a handover candidate celllist of handover candidate cells, corresponding triggering condition foreach handover candidate cell on the handover candidate cell list, and anearly handover indication; sending the early handover command to the UE;and preparing one or more handover candidate cells for a UE-triggeredhandover based on one or more triggering events.
 12. The method of claim11, wherein the preparing for a UE-triggered handover involvesperforming sequence number (SN) STATUS TRANSFER to the handovercandidate cells.
 13. The method of claim 11, wherein the preparing forthe one or more candidate cells further comprising: sending aCONDITIONAL HANDOVER REQUEST to each of the candidate cells; andreceiving corresponding CONDITIONAL HANDOVER RESPONSE from the one ormore candidate cells.
 14. A user equipment (UE) comprising: atransceiver that transmits and receives radio signals in a wirelessnetwork; a measurement report circuit that receives an early measurementreport configuration that configures UE early measurement reports from aserving cell; a handover command receiver that receives an earlyhandover command from the serving base station, wherein the earlyhandover command comprising at least one early handover elementscomprising a handover candidate cell list of handover candidate cells,corresponding triggering condition for each handover candidate cell onthe handover candidate cell list, and an early handover indication; acondition monitor that monitors triggering conditions for candidatecells on the handover candidate cell list based on a UE-triggeredhandover configuration; and a handover circuit that upon receiving anetwork-triggered handover command to a target cell suspends theUE-triggered handover configuration.
 15. The UE of claim 14, wherein themeasurement report circuit sends an early measurement report upondetecting one or more reporting conditions comprising detecting a cellquality of a neighboring cell being higher than a predefined qualitythreshold and detecting a cell quality of a neighboring cell being apredefined offset higher than a cell quality of the serving cell. 16.The UE of claim 15, wherein the UE operates in a multi-beam network, andthe cell quality is determined be over a threshold by meeting at leastone condition comprising a weighted average of qualities of some or allof beams being over the threshold, and a predefined number of beams withcorresponding measurement being above the threshold.
 17. The UE of claim14, wherein the handover circuit performs a random access to thecandidate cell; and sends a handover complete message to the servingcell upon success of the random access.
 18. The UE of claim 14, whereinthe handover circuit stores the UE-triggered handover configuration uponreceiving the early handover command, wherein the UE-triggered handoverconfiguration comprising the handover candidate cell list of handovercandidate cells, and at least one of corresponding cell identification(ID) of each candidate cell, corresponding C-RNTI, and correspondingsecurity algorithm identifier of each candidate cell
 19. The UE of claim14, wherein the handover circuit discards the UE-triggered handoverconfiguration upon a successful completion of a network-triggeredhandover to the target cell, and wherein the UE resumes the suspendedUE-triggered handover configuration upon a failure of anetwork-triggered handover to the target cell.
 20. The UE of claim 19,wherein the handover circuit performs a UE-triggered handover to acandidate cell on the handover candidate cell list when correspondingone or more triggering conditions for the candidate cell are satisfied.